Motor oil is also used for micro-sealing between pistons, piston rings and cylinder walls, as there is always a gap of several microns between these parts. Thanks to the additional sealing effect of the oil, the high gas pressure resulting from combustion is transferred without losses to the piston crown and converted into kinetic energy. Thus, optimum engine power is achieved.
Another function of engine oil is additional engine cooling. When fuel is burned, only a quarter of the released energy can be used for forward movement, part of the energy leaves the engine with exhaust gases, the remaining part of the thermal energy must be removed by the coolant and engine oil. For example, the piston can only be cooled by engine oil. On the one hand, the heat from the pistons is transferred through the piston rings and sealing oil to the working surfaces of the cylinders, then to the coolant. And on the other hand, the oil sprayed in the crankcase directly cools the pistons. Accordingly, each bearing of the crankshaft, camshaft, etc. is cooled by the flow of oil passing through it. The heat accumulated by the engine oil is transferred through the walls of the oil pan to the air flow flowing around it.
All-season oils
Modern oils are refined lubricants from petroleum. But before the refined lubricant turns into motor oil, oil manufacturers add special additives to it, which at the end of the production process make up to 20% of the oil formula.
Additives protect the oil from oxidation and prevent foaming at high engine speeds. Some of the most important additives are viscosity index improvers, which are long-molecule chains that swell when heated and shrink again when cooled. These additives work in such a way that the oil adapts to the engine operating temperature and covers several viscosity classes at once. These additives also have a negative property: at high temperatures, they lose most of their properties. In addition, water, fuel and combustion residues set the limit of the service life of motor oil. Mineral oil does not withstand the pressures and temperatures prevailing in the engine for long enough. Therefore, regular oil changes are not a luxury that can be dispensed with, but a necessary standard that ensures the normal and long-term operation of your engine.
Synthetic oils
Synthetic oils are considered the most expensive, as they are partly made up of artificial chemical compounds of carbon, hydrogen and oxygen. Only those molecules are selected that can optimally perform the required lubrication tasks. In practice, this means that these oils age much more slowly than mineral oils. They also evaporate more slowly and have high long-term stability, meaning that they retain their original highest viscosity grade for a long time.
Purchasing motor oil
Regular all-season oil is suitable for the engine of your Fiesta. It is only important that the oil meets all existing standards. In case of doubt, find out the information you are interested in from the seller, without paying special attention to the powerful advertising campaign of the oil manufacturer. The only evidence of the applicability of the oil is the oil specification, the correct viscosity class and the text printed on the packaging.

Fig. 5.2. Range of use of motor oils of different viscosity in gasoline engines depending on the outside air temperature
Ford recommends SAE 5W-30 motor oils (SAE 10W-40 for diesel engines) that meet the ACEA A1-96, B-96 specification or the Ford WSS-M2C912-A1 specification (for ACEA A3-96, B3-96 diesel engines) (Fig. 5.2, 5.3). If you want to use another oil, it must meet the API SH quality for gasoline engines (for diesel engines CC MC G5, PD2, API SH). Oils with the API SC, SD, SE or SF designation may even impair the normal operation of your engine. If the oil meets the specified criteria, oil grades from different manufacturers can be mixed with each other. You should only take into account that the special properties of the previous formulations may deteriorate over time. Each oil grade has a special additive formulation, the effect of which may deteriorate when mixed with other oils. For this reason, mixing mineral and synthetic oils is also not beneficial.

Fig. 5.3. Range of use of motor oils of different viscosity in diesel engines depending on the outside air temperature

TECHNICAL DICTIONARY
Basic concepts. Oil classification systems
Viscosity.
This is a value for determining the fluidity of the oil. In winter, the engine oil should be so thin that after starting a cold engine, it immediately reaches all lubrication points. On the contrary, in summer, thick oil should be used so that the oil film does not break at high temperatures.
SAE (Society of Automotive Engineers).
Oil fluidity is indicated by viscosity class. The corresponding classes are established by the American Institute of the Society of Automotive Engineers (SAE), for example, SAE 5W-30. The lower the first number, the easier the oil flows at low temperatures (W – winter). 0W oil flows at a temperature of –30°C, at 5W this figure increases to –25°C, at 15 W – to –15°C. The higher the second number, the better the oil withstands high temperatures.
ACEA (Association des Constructeurs Européen dґAutomobiles).
A European standard for oil introduced in 1996 that replaced the CCMS standard. For petrol engines, there are groups A1 (not containing rectified alcohol), A2 (with a small content of rectified alcohol) and A3 (energy-saving oil). For diesel engines, oils are divided into groups B1, B2 and B3.
CCMC (Comité des Constructeurs dґAutomobiles du Marché Commun).
The specification consists of the letters "G" (petrol) and "PD" (diesel) and a number. The higher the number, the better the quality of the oil.
API (American Petroleum Institute).
The specification consists of the letter "S" (petrol engine) or "C" (diesel engine) and a subsequent letter. The further down the alphabet the letter is, the better the quality of the oil.
For this reason, you should not mix pure diesel oils with petrol engine oils, or in the worst case, you will cause serious damage to your engine.
The Fiesta's petrol engines do not require expensive synthetic oil. Ford recommends using SAE 5W-30 viscosity grade oil in our latitudes (see Fig. 5.2).
Oil consumption
If your Fiesta's engine is in excellent technical condition, you should add a small amount of oil (0.25-0.5 l) between the prescribed oil change intervals, and the diesel engine is no exception. This rule applies to all engines only if you change the oil regularly and your engine is not overloaded with an excessively sharp, sporty driving style. Any engine uses a little oil. Some of the engine oil burns during engine operation, which is normal. Leaky spots in the engine, damaged valve timing gaskets, worn oil scraper rings, excessively large clearances between the pistons and cylinder running surfaces, or excessive clearance between the valve guides and valve stems dramatically increase normal oil consumption.
Blue exhaust plume is an indicator of wear
By the blue exhaust plume you can directly identify one of many engine faults. Even when the oil level remains unchanged, this does not indicate a normal engine condition: excess fuel and condensed water dilute the oil, which leads to a deterioration in its lubricating properties. This phenomenon is observed primarily in winter and when driving short distances - the engine rarely reaches its operating temperature, and moisture does not have time to evaporate from the oil. In this case, it is necessary to change the oil at shorter intervals: after 6,000 km of run or once every six months.
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